The present invention relates to an instrument for measuring the displacement or the velocity of an object relative to a reference surface. In general terms, this instrument includes a rotating disc on which is formed an optical, spirally striped pattern, and which is mounted either on the reference surface or to follow the movement of the object. A scale, having an optical pattern of stripes preferably arranged perpendicular to the direction of movement of the object, is mounted either to follow the movement of the object or on the reference surface. By means of an optical arrangement an image of one of the striped patterns is projected onto the other striped pattern, and a photodetector is provided for measuring the light passed by the other striped pattern. Means are provided for generating a reference signal which is applied as an input to a circuit for evaluating the measuring signal provided by the photodetector and comparing it to the reference signal.
In one device of this type known to the art, the optical, spirally striped pattern consists of a long, multiply winding translucent spiral in an otherwise opaque disc (German Pat. No. 1,177,353, FIG. 4). The pitch of the spiral is constant and corresponds to the spacing of straight, parallel bars of the striped pattern of a scale arranged near the disc. In order to properly orient the two striped patterns, at least in a small zone of intersection between the two patterns, the stripes of the pattern on the scale are arranged tangentially to the stripes of the pattern on the disc in the intersection zone and thereby substantially perpendicular to the radius of the disc which passes through the intersection zone. When the scale is stationary with respect to the center of rotation of the disc, the two striped patterns intersect to pass light to a photodetector once with every revolution of the disc. The frequency of the measuring signal given off by the photodetector is therefore equal to the rotation frequency of the disc.
This often results in a relatively low frequency measuring signal, which can be an important disadvantage when circuits are used for evaluating the measuring signal in which the accuracy of the result is substantially proportional to the frequency of the measuring signal. This is often the case, for example, when digital circuits are used to compare the number of wave trains of the measuring signal and of a reference signal. With devices of this type, the disc must be driven at a very high velocity in order to achieve high accuracy, in practice resulting in a device which is often expensive to manufacture.
In another known device of the prior art the stripes of the striped pattern of the disc are radially arranged (German Pat. No. 1,177,353, FIG. 2). A scale with a striped pattern having straight, parallel stripes is arranged near the disk and oriented such that the scale stripes run parallel to a radius of the disc passing through the midpoint of the image zone allocated to the photodetector. Here, the frequency of the measuring signal is a multiple of the frequency of the disc, corresponding to the number of radial stripes, and it is therefore sufficiently high for the previously mentioned circuits for the evaluation of the measuring signal. An important disadvantage here, however, is the fact that a complete match between the stripes of the two striped patterns is not possible, since the radial stripes which do not pass through the center of the image zone form an angle to the parallel stripes of the striped pattern of the scale which increases with distance from the center of the zone. Therefore, when an image zone large enough to produce a sufficiently large measuring signal is used, a measuring signal with a high interference signal constituent is often obtained.